'Until not much more than 20 years ago, economists frequently lamented the fact that they were limited in their empirical analyses to statistical assessments of market behavior, because controlled economic experiments were (thought to be) infeasible, unethical, or both. Much has changed in the...

Economists have a long tradition in identifying the evolution of cooperation in large, unstructured societies as a puzzle. We suggest a new explanation for cooperation which avoids restrictions of most previous attempts. Our explanation deals with the role of internalized norms for cooperation...